I just discovered this by accident and haven't seen it mentioned anywhere before:

With a finder window in column view, select an application so that the info and large icon for the app shows up in the right most column. You can then drag and drop documents onto the application's large icon in the info column to open them in that app.

Maybe this hint has gone unreported because it's hard to imagine it being very useful!

The preview "picture" for anything (except where media is previewed a-la QuickTime) seems just to be a big icon. Try double clicking a preview picture to launch it, or drag it on to the desktop to copy it etc.

Pretty useless IMHO, unless you're in a REALLY big desktop and the actual icons are too small to click ;)